I. Field of the Invention
This invention relates in general to rotary well drilling, particularly to shock absorbing apparatus used in the drill string to minimize vibrations transmitted from a drill bit, through the drill string and to the equipment at the surface of the earth.
II. Description of the Prior Art
Shock absorbing apparatus used in the drill string of a rotary well drilling apparatus may be classified into two types: (1) Shock absorbers for oil well drilling and (2) shock absorbers for hole drilling for other industrial purposes such as blast hole drilling.
Representive of shock absorbers used in drilling oil and gas wells may be seen in U.S. Pat. Nos. 3,382,936 and 3,746,329. In these shock absorbers a gas cavity is formed between a mandrel and a tubular body for supporting the load imposed upon the drill bit and for absorbing any shock loading or vibrations that would otherwise be transmitted between the drill bit and the equipment at the surface of the earth. Due to the large hydrostatic pressure of the liquid drilling fluids used to drill deep oil and gas wells, it would be difficult to seal between this large hydrostatic pressure and the much lesser pressure of the gas inside the apparatus. To solve this problem there is disclosed in U.S. Pat. No. 3,382,936 a pressure transmitting liquid chamber which communicates with the gas cavity inside the apparatus and the ambient drilling fluid. As a consequence, the liquid in the intermediate liquid chamber may be sealed much easier than the gas due to the relatively high viscosity of liquid when contrasted with gas. The liquid and gas inside the apparatus is prevented from intermingling by use of a movable and fluid responsive separation element, which in U.S. Pat. No. 3,382,936 was a flexible membrane or bag. When problems develop with this flexible membrane or bag, a solid piston was invented to maintain separation between gas and liquid, as is disclosed in U.S. Pat. No. 3,746,329. Subsequently, there was disclosed another apparatus to accomplish results similar to the above two apparatuses, and in addition a reduction of torsional shock loadinds and vibrations, as may be seen with reference to U.S. Pat. No. 3,998,443.
With respect to the industrial type shock absorber, there is disclosed in U.S. Pat. Nos. 4,055,338 and 4,145,034 a shock absorber particularly adapted for use with a blast hole drill rig. Blast hole drill rigs are utilized to drill shallow holes, approximately fifty feet deep, for lowering and detonating explosives to disintegrate the earth for mining. The shock absorber disclosed in the above two patents is adapted to be placed in the drill string at a location near the surface of the earth. As is the case with shock absorbers used in oil and gas well drilling, gas is utilized for carrying the load imposed upon the drill bit and the shock loading or vibrations transmitted from the drill bit to the surface of the earth and the equipment used to motivate the drill bit. Since air is used as the circulating medium to remove cuttings from the bottom of the bore hole, there is no need for an intermediate or pressure transmitting liquid chamber inside such shock absorbers. Rather, there are seals between the mandrel and tubular body to seal the gas inside the apparatus and prevent lubricant from exiting from the apparatus. The lubricant is necessary in order to provide adequate lubrication to the seals. In the apparatus disclosed in U.S. Pat. Nos. 4,055,338 and 4,145,034 the pressure of the lubricant in the lubricant cavities is maintained intermediate the pressure of the pressurized chamber and ambient. This is beneficial in that none of the seals is exposed to a pressure differential as large as the differential between the pressure inside the pressurized chamber and atmospheric pressure. Hence, the seals are exposed to less stress and deformation and can be expected to have a longer life. In these devices a selected pressure is applied to the lubricant inside the lubricant cavities by injecting lubricant through a grease fitting. The pressure in the lubricant cavities is independent of the pressure in the load transmitting and shock absorbing chamber. In such devices, because of depletion of lubricant, the lubricant pressure will eventually drop. After about eighty to one hundred hours of operation, relubrication is necessary and failure to relubricate can result in substantial damage to the seals and to the shock absorber.